The document WO 02/47441 (Hu) discloses an abovementioned circuit arrangement. In this document, two PFC technologies are disclosed: a charge pump (“single feedback”) and a so-called valley fill circuit (“double pump”). Here, the charge pump is connected upstream of the valley fill circuit, when seen from the system voltage. In addition, the charge pump and the valley fill circuit are supplied by the same radiofrequency voltage source.
Charge pumps are known, for example, from the document U.S. Pat. No. 4,949,013 (Zuchtriegel). The essential feature of a charge pump is the connection of a rectifier output to a pump diode. A radiofrequency voltage which is drawn from a load circuit is applied to the connection point formed. At this point it should be emphasized that only one rectifier output is connected to a pump diode, for which reason this part of the circuit arrangement is referred to as “single feedback” in WO 02/47441 (Hu).
Valley fill circuits are known, for example, from the document WO 90/09087 (Skalak). Of concern here is a passive PFC circuit comprising two storage capacitors and three diodes which is connected between two rectifier outputs. The operation of the valley fill circuit is based on the fact that the storage capacitors are connected via the diodes such that they are charged by a system voltage as a series circuit but are discharged by a load as a parallel circuit.
A valley fill circuit may also act, to a limited extent, as a charge pump by one of its diodes being split into two diodes. For this purpose, a radiofrequency AC voltage needs to be applied to the connection point between the two diodes resulting from the splitting. The document U.S. Pat. No. 6,316,883 (Cho) describes a valley fill circuit which has been modified in this manner. The operating device described there for discharge lamps in addition also has a separate charge pump. This charge pump is connected downstream of the valley fill circuit when seen from the system-voltage side, as a result of which a further storage capacitor is required.
In WO 02/47441 (Hu), the charge pump is connected upstream of the valley fill circuit. A further storage capacitor is thus not required. The radiofrequency AC voltage, which is supplied to the charge pump, is derived from the radiofrequency AC voltage which is fed to the modified valley fill circuit.
It can be understood from the document WO 02/47441 (Hu) that the circuit arrangement described there achieves good values for the power factor. Standards such as IEC 61000-3-2, however, prescribe in addition limit values for rated current harmonics. Here, a distinction is drawn between light sources which draw up to 25 W from the system voltage and light sources which draw more than 25 W. Above 25 W the specifications are substantially higher, i.e. the amplitudes of the rated current harmonics need to be substantially lower.
One subject matter of the document WO 02/47441 (Hu) is a compact fluorescent lamp having an integrated operating device. Such lamps are customary on the market up to a power consumption from the power supply system of 25 W. Since, up to 25 W the specifications in relevant standards for the rated current harmonics are low, the circuit arrangement disclosed in WO 02/47441 (Hu) can provide fluorescent lamp operation which conforms to standards up to 25 W.
There are many specifications for a circuit arrangement for operating light sources. The following specifications should be taken into consideration when designing these circuit arrangements:                low system-side power factor        low total harmonic distortion (THD) of the current drawn from the power supply system        rated current harmonics which conform to standards        high efficiency        low crest factor of the current through the light source        low radio interference        low costs        small geometric dimensions        
In order to operate fluorescent lamps at a power consumption from the power supply system of up to 25 W, the circuit arrangement disclosed in WO 02/47441 (Hu) is a good compromise for meeting the abovementioned specifications. Above 25 W, however, it becomes problematic to conform to the relevant standards as regards rated current harmonics. In particular for fluorescent lamps, the crest factor of the lamp current is limited by standards (for example IEC 60929) to a maximum value of 1.7. It is also problematic to maintain this limit value given a power consumption from the power supply system of over 25 W.
Dimensioning the circuit arrangement disclosed in WO 02/47441 (Hu) such that the standards as regards the rated current harmonics are adhered to even at a power consumption from the power supply system of over 25 W means that there is a considerably increased load on components in the circuit arrangement. This results in an increase in the costs, in greater geometric dimensions and in reduced efficiency.
If it is necessary also to adhere to the limit value for the crest factor of the lamp current according to IEC 60929, the components will be subject to an even greater load.